The present invention relates to residential heating and/or cooling systems and other indoor comfort systems, and is more particularly concerned with battery-powered thermostats of the type that derive the power for monitoring conditions within the comfort space and for controlling the signaling to the furnace or other comfort system from a battery i.e, dry cells or alkaline cells. The invention is more specifically directed to DC thermostats with a battery level monitoring function, and with a display that includes a low-battery alert message when the energy remaining in the battery is low and the battery should be replaced. Heat may be provided from a gas, oil, or electric furnace or heat pump, and cooling may be provided from a compression/condensation/expansion/evaporation cycle air conditioner, an absorption type air conditioner, a ground-water heat exchange cooing system, or other available chilling apparatus. As used here, the term “cooling” includes both sensible cooling (reducing the temperature of the comfort air) and latent cooling (removing humidity). These comfort air apparatus may have additional functions for better control of the environmental air in the comfort space, such as multiple fan speeds, high and low heating, and high and low compressor speeds.
Wall thermostats are typically installed on an interior wall of a dwelling, business space or other residential or commercial space to control the operation of a furnace, air conditioner, heat pump, or other environmental control equipment. The thermostat continuously monitors the temperature of the room or other interior comfort space or zone, and is connected by a run of thermostat wires to the associated environmental control equipment to signal a call for heating, a call for cooling, or otherwise to keep an interior comfort space parameter, such as temperature, within some range (e.g., 68° F., ±1.0° F.). Other controls may be available, sensitive to other parameters, such as humidity or particulate level.
In many applications, i.e., in many permanent homes, and in mobile, recreational, or marine dwellings and spaces, battery-powered thermostats are frequently the preferred thermostat. These typically have a battery power source, e.g., a pair of AA alkaline power cells, installed within the thermostat housing to power the electronics. These supply DC energy to the internal electronics within the thermostat and power latching relays (or in some cases, a solid-state switch, such as a high-side switch, triac switch, or digital signal) that connect thermostat power (such as 24 volts AC) to the particular thermostat wires that control heating, fan, air conditioning, and so forth. As used in this description and in the accompanying claims, the term “latching relay” should be read to include both electro-magnetic latching relays and also equivalent solid state switching devices such as the afore-mentioned high-side switch or triac switch, etc. The thermostat is constantly monitoring the temperature in the comfort space and is also constantly monitoring the voltage level available on the DC battery power source. One or more controls, i.e., push buttons, rotary knob(s) etc., allow the occupant to set and adjust temperature setpoints for heating and/or for air conditioning. An LCD display screen on the housing of the thermostat allows the occupant to see the room temperature and also to see other functions, such as temperature setpoints during a temperature setting sequence. A low-battery message can be displayed on this screen if the battery voltage drops below a level that indicates the battery is approaching the end of its useful life and should be replaced.
It is frequently the case that the occupant has no reason to check the thermostat or look at it. If this is the case when the battery voltage drops, it may be that the low-battery message will appear on the thermostat display screen and the occupant will not notice it for a period of several weeks. However, normal heating and/or cooling operations mean that the thermostat will continue to draw power from the power cells, especially when the thermostat pulses the latching relay when there is a call for heat or cooling and again when the thermostat is satisfied. The remaining battery energy continues to be consumed and the battery voltage will drop; eventually the thermostat will stop operating if the batteries are not replaced with fresh ones. It also may be the case that a low-battery condition will occur when the occupant is away for an extended period, and that the energy cells may reach the end of their life before the occupant returns. If the furnace or other HVAC apparatus fails to cycle properly during winter conditions, this may result in frozen water pipes or other damage.
Because of the foregoing problems, it would be desirable to provide a feature or features for battery powered thermostats that will prolong the battery life when a low-voltage condition occurs, and if possible change the heating and/or cooling conditions in the comfort space in a way that will gently induce the occupant to check the thermostat (and thus be alerted to the low-battery message displayed there).